Surge-protected fuse link



p 1939- R. R. PITTMAN ET AL 2,174,477

SURGE-PROTECTED FUSE LINK Filed 001:. 29, 1938 INVENTORS Cam/ MugPatented Sept. 26, 1939 PATENT OFFICE SURGE-PROTECTED FUSE LINK Ralph B.Pittman and Carroll H. Walsh,

. Pine Bluff, Ark.

Application October 29, 1938, Serial No. 231,750

1 claim (or. 200-115) 'This invention relates generally to protectivedevices for alternating current electrical circuits, and particularly tofusible devices used to prevent the passage of excessive amounts ofelectrical en- 6 ergy through said circuits.

Many circuit-interrupting devices, such as the expulsion fusesillustrated in Patent No. 2,109,090, dated Feb. 22, 1938, and issued toRalph R. Pittman et al., depend for their operation upon the i fusing ofa fuse link in response to the passage therethrough of a predeterminedalternating current for a predetermined time. When such devices areconnected to overhead electrical circuits which are exposed to lightningdischarges, the superimposed or surge currents attending such dischargescause the fuse links to fuse while. the alternating current through thefuse link maybe well within the rating of the link. The energytransmitted by the surge current through the fuse link is the result ofa relatively high current for a very short time, so that the voltageportion ofthe surge energy around the fusible' element and thus limitingits temperature rise to a value below the fusing temperature. It is anobject of the present invention to provide means for shunt-circuitingthe fusible element of a fuse link forming a part of a circuit uponwhich is impressed a high superimposed current of surge characteristics,such shunt-circuiting preventing the heating of the fusible element tothe fusing temperature, together with means for eifecting the desiredby-passing through an auxiliary circuit without affecting the desiredprotective'features of the fuse link under excessive current of a normalalternating 40 character. Other objects will in part be obvious,

and in part pointed out hereinafter.

The purpose of the present invention is in some respects similar to thatset forth in our copend- 'ing application Serial No. 230,076, filedSept. 15,

1938, but the structural .distinctions 'herein will be quite apparent. I

The invention comprises the elements and combinations thereof, featuresof construction, and arrangement of parts exemplified in the followingdescription, and the scope of tie invent on will be indicated in theaccompanying claims.

In the drawing: Fig. l is an elevational view of the fuse link; Fig. 2is an enlarged view of the fusible element assembly, shown partly inheretofore been well recognized. On the other hand, should thedisturbance be sufficiently se vere to cause failure of insulation inconnected apparatus, the abnormally excessive alternating current followcurrent should be promptly interrupted by the fusing of the fusibleelement of the protective device.

With the foregoing requirements in mind, reference is made to thedrawing illustr'ating the invention. The fuse link includes the axiallyspaced longitudinally extending conducting members II and H, which areelectrically connected and mechanically Joined by the fusible heating 25element II, the upper end of the latter being joined to the member H bymeans of the upper fuse element terminal l2, into which the respec tivemembers extend and are die-pressed. For convenience in connecting thefuse link in an ex- 30 pulsion fuse tube, a metal terminal button [0 issecured at the upper end of the conducting member II. The lower end ofthe fusible heating element l8 extends into a. downwardly extendingrecess in the lower fuse element terminal I 6, and 35 these elements arejoined by means of a layer of low melting point solder l9, placedtherebetween. The conducting member l1 extends upwardly into a centrallypositioned recess in the lower fuse element terminal l6, and isconveniently die-pressed therein.

To reduce arcing time following fuse blowing, the fusible heatingelement I8 is housed within a hollow tube of insulating material 20,which is conveniently formed ofhard fiber, the respective ends of whichare closed by but not fastened to the fuse element terminals l2 and [6.

In the fuse link just described, the point of rupture of the fusibleelement is related to the rate of increase in excess current. A smallover- 59 load current will eventually heat the solder I9 to its melting'point, while a relatively high current may fuse the heating elementwithout melting the solder IS. The latter is preferably abismuthtin-lead alloy, and the material of the fusible 55 heatingelement is conveniently a nickel-chromium-steel alloy.

The fuse element terminals l2 and I6 are each provided with axiallyaligned outwardly extending integral flanged portions, and a stressedhelical metal spring I3 engages the lower surface of the flange of theupper fuse element terminal l2 and extends downwardly therefrom alongthe outer surface of the tube 20 to a point near the flange of the lowerfuse element terminal I6. A perforated or apertured washer of insulatingmaterial l5, preferably of mica, which is provided with a plurality ofcircumferentially spaced apertures 24, is positioned upon the uppersurface of the flange of the lower fuse element terminal 16, encirclingand in coaxial relation with the tube 20, and a metal washer I4 isinterposed between the lower end of the spring l3 and the upper surfaceof the apertured washer IS, the arrangement being such that the springl3 urges the metal washer l4 against the insulating washer l 5, which inturn is urged against the flange of the lower fuse element terminal l6.

From the above description, it will be seen that the metal washer M andthe lower fuse element terminal it form the respective electrodes of aelement to the metal ectrode er r i l, and further, that the voltageimpr" upon the spark gap is the voltage requi 1 cause a ct tent to flowthrough. the fusible nating element ;or best o eration, is desirable thethe mal capacity of tits fusible heating element be not less than300-500 crest amperes when subjected to a current wave rising to crestin 1% micro-seconds, and also g to half value in ac micro-seconds, andme resistance of the fusible heating element e such that crest voltageof the order of volts appear across the 1 it passes For exampere linkhaving a surge current capacity of 600 am oeres, and a resistance of 5ohms. Under such a set of conditions, a c nt of 300 amperes causes avoltage drop of 00 volts for breaking down the spar: gap the by-passcircuit, thus providing a factor of safety of 2. Similarly, anotherpractical construction embodies a nominally rated 1 ampere link having asurge capacity of 2000 amperes, and a resistance of 1 /2 ohms, which ergization provides a similar factor of safety in preventing the fusing ofthe link,

in the event the invention is connected in a circuit to apparatus theinsulation of which falls coincident with the spark over of the sparkgap, the are voltage maintained by the alternating current followcurrent will be impressed across the fusible heating element. This arevoltage is ample to cause sufficient current to be maintained throughthe i'usible'heating element to cause the melting of the solder iii,permitting the spring to urge the conducting members apart and thusinterrupt the circuit. As the conducting member l r the conductiu memberth spark gap more alo g that changes and modifications may be madetherein without departing from the spirit and scope of our invention,and we desire that our invention be limited only as is necessitated bythe .prior art and the accompanying claims.

We claim, as our invention:

1. In a fuse link, longitudinally spaced conducting members, a hollowtube of insulating material, a fusible element within said tubeelectrically connecting said conducting members, a helical metal springurging the separation of said conducting members, said helical springbeing electrically connected to one of said conducting members, andelements constituting a discharge gap interposed between the other ofsaid conducting members and said spring,

2. In a fuse link, axially spaced longitudinally extending conductingmembers, a hollow tube of insulating material interposed between theadjacent ends of said conducting members, a fusible element within saidtube which electrically con.- nects and mechanically joins 52 dconductin members when said element is up u cally conducting resilienturge said conducting members t in response to the fusing of said fusiblec-m it, means electricaliy connecting said re e t means to one of saidconducting members, o. ielectric means responsive to a predeterin vo ageinterposed between said resilient ine the other of said conductingmembers 3. in a fuse link, longs ducting members, a hell terial disposedthereb sible element within said tube electric l ting and mechanicallyjoining the adgacent e ds cf said conducting members when F -c elementis unfused, conducting resll s urging said conducting elements apart,electrically connecting said resilient mea .e said conducting members,and (eel interposed between said resiliei other conducting member.

In a fuse link, longitudinally spaced con-- ducting members, a hollowtube of insulating material disposed therebetween, a fusible elementwithin said tube, and helical metal spring mounted externally of saidtube, elements constituting a spark gap positioned adjacent one of saidconducting members, and means including said metal spring forelectrically cormecting said spark gap in shunt-circuit relation withsaid fusible element.

5. A fuse link comprising axially spaced conducting members, fuseelement terminals secured to the adjacent ends of said conductingmembers, a hollow tube of insulating material bridging said terminals, afusible element within said. tube electrically connecting saidterminals, conducting spring means engaged with one of said terminalsand extending along t'f'ic e said i tube toward the otne a metal washerencircling said tube '0 1 anged to lly spaced conisulating maextendedend of said spring means, and a washer of insulating material havingtherein a plurality of circumferentially spaced apertures extendingtherethrough in the direction of the axis of said tube, said washer ofinsulating material being engaged on one side by said metal washer andon the other by a portion of said other terminal.

6. A fuse link comprising axially spaced conducting members, fuseelement terminals secured to the adjacent ends of said conductingmembers, a hollow tube of insulating material bridging said terminals, afusible element within said tube electrically connecting said terminals,conducting spring means engaged with one of said terminals and extendingalong said fuse tube towardthe other terminal, a metal member engagedwith the extended end of said spring means, and an element of insulatingmaterial having therein an aperture extending therethrough in thedirection of the axis of said tube, said element of insulating materialbeing engaged on one side by said metal member and on the other by aportion of said other terminal.

7. A fuse link comprising axially spaced conducting members, resilientmeans of conducting material arranged to urge said conducting membersapart, a fusible element electrically connecting said members andrestraining the movement thereof, means electrically connecting saidresilient means to one of said conducting members. and normallynon-conducting means responsive to an abnormal current of a transientcharacter 7 through said fusible element interposed between rent.

RAIJPH R. PII'I'MAN. CARROLL H. WALSH.

